Trauma to the elbow joint frequently involves damage to the ligamentous support of the elbow and fractures of the osseous structures responsible for the skeletal integrity of the elbow joint. The proximal aspect of the radius, or radial head, is frequently injured either in isolation or in combination with injury to other bony or ligamentous structures of the elbow joint. The radial head may also be fractured in association with injuries to the forearm axis, including disruptions of the interosseous membrane between the radius and the ulna. Whether in isolation or in combination with other injuries, fractures of the radial head can be difficult to treat.
Fractures of the radial head are either reconstructable or unreconstructable. Despite various technical advances in the reconstruction of radial head fractures, a certain percentage of fractures are not amenable to reconstruction due to the degree of comminution or severity of the fracture. In general, unreconstructable radial head fractures result from high energy trauma and are therefore frequently associated with significant injuries to other osseous or ligamentous structures of the elbow joint or forearm. In these cases, restoration of the stabilizing function of the radial head is critical to allow the ligaments of the elbow or forearm to heal in appropriate relationships, thereby restoring stability to the elbow or forearm. This stabilizing function depends, in part, upon re-establishing the appropriate distance between the capitellum and the proximal shaft of the radius.
Prosthetic replacement of the radial head has evolved rather slowly. The first widely used prosthetic radial head was introduced in the 1970's and was composed of silicone. Silicone implants placed in various joints throughout the body led to “silicone synovitis,” in which the silicone induced an inflammatory response within the joint. Further, silicone radial head prostheses were found to be incapable of resisting the stresses to which the radial head is subjected, rendering it less useful in stabilizing the injured elbow or forearm.
The difficulties apparent with silicone led to experimentation with metal radial head implants. These prostheses are fashioned from a single piece of metal (often titanium) and include a stem and a head portion. The head portion is shaped to approximate the anatomy of the radial head. These metallic prostheses are capable of resisting the compressive stresses to which the radial head is subjected, as has been demonstrated in several biomechanical studies. However, significant problems remain with these prostheses.
Anatomic and radiographic studies of the dimensions of the radial head reveal a disparity with currently available metallic prostheses. Therefore it has been difficult to restore appropriate anatomic alignments within the elbow. Therefore restoration of the appropriate relationship between the capitellum and proximal shaft of the radius has been very difficult to achieve with these prostheses. Additionally, the fact that these prostheses are fashioned from a single piece of metal has led to technical difficulties with insertion and removal. Surgeons have had difficulty with matching both the size of the stem to the canal of the proximal radius and the size of the head portion to the patient's native radial head. Removal of these non-modular components frequently requires release of the lateral ligaments of the elbow and the annular ligament, which binds the neck of the proximal radius to the proximal ulna. Thus the elbow is frequently destabilized during removal of these prostheses.
Designers of prosthetic joint replacements in the hip, shoulder, knee and fingers have circumvented the above mentioned difficulties by employing the use of modular components. Modularity allows for each aspect of a prosthesis to be sized appropriately to its recipient anatomic site. The concept of modularity has only recently been applied to commercially available radial head prostheses. Currently available modular radial head prostheses employ a mechanism by which the head component is impacted over and onto the stem component. The surgical exposure must therefore allow sufficient room for the head to be maneuvered over the stem prior to being impacted. With impaction, the height of the prosthesis may be decreased, resulting in an increased distance between the capitellum and the proximal end of the radius. Increasing this distance alters the bony anatomy such that the ligaments of the elbow joint are not held in their appropriate lengths and tensions. Instability of the elbow or inappropriate healing of the ligaments may result. Furthermore, removal of these prostheses is accomplished in the same manner as the above mentioned metallic implants, often requiring destabilization of the lateral aspect of the elbow joint.
In order to reap the benefits of modularity in radial head prosthetic replacement, a reliable and surgically appropriate method to secure the stem of the prosthesis to the head of the prosthesis and which allows for accurate restoration of the appropriate spatial relationships between the bones of the elbow is required.